Immunocytochemical studies of tryptophan hydroxylase, tyrosine hydroxylase, and serotonin innervation in the aging rat neurointermediate pituitary.

Innervation within the aging male rat pituitary neurointermediate lobe was examined using immunocytochemical methods with antisera to rate-limiting enzymes for serotonin (tryptophan hydroxylase, TpOH) or catecholamine (tyrosine hydroxylase TH) synthesis. Immunostaining for both enzymes was observed in both neural and intermediate lobes in younger rats (age 6 weeks or 7 months), but TpOH was absent from the intermediate lobe in older groups (10-12, 15-16, and 18-19 months). Colocalization of TpOH and TH was demonstrated in some nerve fibers, using immunofluorescence. The oldest age group showed a reduced staining pattern for TH. 5-hydroxytryptophan, the immediate precursor for serotonin (5-HT), was taken up and converted to immunoreactive 5-HT in all age groups. Overall, the pattern of staining for regulatory neurotransmitters or enzymes which ultimately affect pituitary intermediate lobe secretory function is significantly modulated during the aging process.